When a power-generating process, such as the Organic Rankine Cycle (ORC), is operated in the environment of another equipment assembly, such as e.g. an internal combustion engine, both the direct integration of the generated energy as mechanical power into the external system (e.g., the expansion machine of the power-generating process can drive the external process at least in a supportive manner), as well as its provision for auxiliaries (e.g., the external process can drive a pump in the power-generating process) are often advantageous, since conversion losses arise when mechanical energy is converted into electrical energy. In addition, costs are also eliminated for the reasons that motors for the drive or generators for the output are omitted, and the compactness can be increased, both of which are critical factors for the integration of a power-generating process into said environment.
However, due to a direct connection (for example, a coupling via a rigid shaft), one of the processes loses the degree of freedom of rotational speed control (usually the downstream process). To avoid this, a connection via a transmission can be effected. As a result, both a stepped and a stepless connection can enable rotational speed control. However, this regain in rotational speed control is accompanied by a number of disadvantageous characteristics. On the one hand, a transmission represents an additional expense which, depending on the application, has considerable influence on the cost-effectiveness. This effect is increased by the fact that transmissions (in particular stepless ones) also lead to a loss of efficiency. Transmissions are also subject to considerable stress and therefore add additional maintenance and related costs to the system. Last but not least, a transmission also consumes a comparatively large amount of installation space, which is contrary to the aim of compactness in many applications of motor integration.
Due to the coupling of the expansion machine presently described or the coupling of both the expansion machine and the feed pump of the ORC system to external processes without a transmission, the degrees of freedom of rotational speed control are lost. As a result, it is not possible to control parameters that are favorable for the ORC operation and that are required for the components—in particular volume flows, temperatures and pressure levels. This poses a particular problem for operations, since the allowed temperatures of the components are limited, in particular on the flow-off side of the expansion machine.
Due to the absence of the rotational speed control of the expansion machine, an expansion ratio can not be selectively provided which correlates to the volume ratio fixedly installed in a volumetric expansion machine. The typical prior art implementation of a variable volume ratio by way of a variable inlet or outlet window represents a complex and expensive process which impairs the cost-effectiveness of ORC systems. However, an expansion which is unsuitable for the expansion machine can lead to greatly decreasing efficiency and therefore likewise to the overall system not being cost-effective, or in extreme cases can result in exceeding the maximum permissible pressure. Exceeding the maximum permissible pressures and temperatures results in the system failing with possible consequential damage.